Embodiments of the present invention relate to a hydraulic tool for moving a first structural element in the axial direction in relation to a second structural element. Embodiments of the present invention also relate to a method for releasing the frictional grip between a cam ring and a number of latching dogs which are moveably mounted in a connector body of a tieback connector.
At an offshore well for oil and gas exploitation, a conduit 1 in the form of a production riser may be connected to a subsea wellhead assembly 2 by means of a tieback connector 3 of the type illustrated in FIG. 1. The illustrated tieback connector 3 is a known type of tieback connector which is not provided with any internal hydraulic cylinders and which is securable to and releasable from a wellhead assembly by means of a separate running and retrieval tool 4. The tieback connector 3 comprises a connector body 5 which is bolted to the lower end of the conduit 1. When the lower part of the connector body 5 has been lowered down over the upper end of the wellhead assembly 2, the tieback connector 3 may be secured to the wellhead assembly 2 by means of the running and retrieval tool 4. The tieback connector 3 comprises a number of latching dogs 6 which are moveably mounted in the connector body 5 so as to be moveable in the radial direction between a retracted unlocking position, in which the connector body 5 is allowed to move axially in relation to the wellhead assembly 2, and an advanced locking position, in which grooves on the inner side of each dog 6 is in engagement with external grooves 7 on the wellhead assembly 2 and thereby prevents the connector body 5 from moving axially in relation to the wellhead assembly 2. Each dog 6 has a tapered wedge surface 8 on its outer side, which is in engagement with a corresponding wedge surface 9 on the inner side of a cam ring 10. The cam ring 10 is fixed to an actuation ring 11 via a number of connecting rods 12. The cam ring 10 and the actuation ring 11 are displaceably mounted to the connector body 5. The running and retrieval tool 4 comprises an annular actuation member 13 and a number of hydraulic cylinders 14, by means of which the actuation member 13 may be moved in the axial direction. When the tieback connector 3 is to be secured to the wellhead assembly 2, the actuation member 13 is brought into engagement with the actuation ring 11, whereupon the hydraulic cylinders 14 are actuated to press the actuation member 13 axially downwards against the actuation ring 11 so as to thereby move the cam ring 10 downwards along the dogs 6. The cam ring 10 will thereby press the dogs 6 radially inwards into engagement with the external grooves 7 on the wellhead assembly 2, as illustrated in FIG. 1. The running and retrieval tool 4 may then be released from the tieback connector 3 and retrieved to the surface. The dogs 6 are maintained in the advanced locking position by the frictional grip between the wedge surface 9 on the cam ring 10 and the wedge surface 8 on the dogs 6. When the tieback connector 3 is to be released from the wellhead assembly 2, the running and retrieval tool 4 is again mounted to the tieback connector 3 and the actuation member 13 is secured to the actuation ring 11 by means of pivotable gripping members 15 on the actuation member 13 which are engaged with a flange 16 on the actuation ring 11. Thereafter, the hydraulic cylinders 14 are actuated to pull the actuation member 13 and the actuation ring 11 axially upwards so as to thereby move the cam ring 10 upwards along the dogs 6 and allow the dogs to move radially outwards into the retracted unlocking position, whereupon the tieback connector 3 may be lifted vertically away from the wellhead assembly 2. However, the frictional grip between the cam ring 10 and the dogs 6 grows stronger and stronger in course of time and could after some years be so strong that the force required for pulling the actuation ring 11 in order to release the cam ring 10 from the dogs 6 is so high that the flange 16 on the actuation ring 11 will be damaged by the gripping members 15 when the hydraulic cylinders 14 are actuated to effect the required pulling force.
A hydraulic tool is known from U.S. Pat. No. 5,273,376 A. This known tool comprises an essentially U-shaped frame which carries hydraulic cylinders. The U-shaped frame is to be inserted into a gap between a riser and a well assembly, whereupon the hydraulic cylinders are pressurized in order to separate the riser from the well assembly.